On 12 November 2014, the Philae lander descended towards comet 67P/ChuryumovGerasimenko, bounced twice off the surface, then arrived under an overhanging cliff in the Abydos region. The landing process provided insights into the properties of a cometary nucleus(1-3). Here we report an investigation of the previously undiscovered site of the second touchdown, where Philae spent almost two minutes of its cross-comet journey, producing four distinct surface contacts on two adjoining cometary boulders. It exposed primitive water ice-that is, water ice from the time of the comet's formation 4.5 billion years ago-in their interiors while travelling through a crevice between the boulders. Our multi-instrument observations made 19 months later found that this water ice, mixed with ubiquitous dark organic-rich material, has a local dust/ice mass ratio of 2.3(-0.16)(+0.2), matching values previously observed in freshly exposed water ice from outbursts4 and water ice in shadow(5,6). At the end of the crevice, Philae made a 0.25-metre-deep impression in the boulder ice, providing in situ measurements confirming that primitive ice has a very low compressive strength (less than 12 pascals, softer than freshly fallen light snow) and allowing a key estimation to be made of the porosity (75 +/- 7 per cent) of the boulders' icy interiors. Our results provide constraints for cometary landers seeking access to a volatile-rich ice sample.